Turbofan engines are frequently employed in aviation. In order for a turbofan engine to be effective in an aviation application, it is typically placed close to other critical portions of an aircraft. For example, a turbofan engine may be mounted on a wing thus placing the turbofan engine in close proximity with both the wing and the fuselage of the aircraft. Therefore, operation of such a turbofan engine must meet certain safety standards.
Safety standards and precautions for turbofan engines are important during all phases of operation, including start-up, shutdown, normal operation, and failure. Safety may be of particular concern during failure of the engine, especially when failure involves the fan itself. One type of failure condition is known as “fan blade off” “Fan blade off” refers to separation of a fan blade from the turbofan shaft. During a fan blade off event, a containment case housing the turbofan engine, specifically around the fan, is expected to prevent the fan blade from travelling along a path that damages the wing or fuselage.
Referring now to the prior art design shown in FIG. 1 a typical turbofan engine 30 is illustrated. The turbofan engine 30 includes a fan containment case 32 surrounding a turbofan 34 and a number of compressor stages. The fan blade(s) 36 are secured to a shaft 38 by way of a rotor disk or hub 40, as seen in FIG. 2, during normal operation. Conventional turbofan engines employ fan blade(s) 36 that are not integral to the rotor disk 40. Instead, the fan blade(s) 36 are individually joined to the rotor disk 40 by dovetail joints. The rotor disk 40 has mounting slots 42 arranged around an exterior surface thereof. A fan rotor with an integral plurality of fan blades permanently connected to the fan disc is often termed a “blisk”, or bladed disc (FIG. 5). This style of fan rotor may be functionally desirable as compared with a conventional style separable fan rotor and is discussed further subsequently.
As seen in FIG. 3, each fan blade 36 includes blade root 44, a blade platform 46, and an airfoil 48. The fan blade root 44 of each fan blade 36 slides into the respective mounting slot 42 such that the root 44 is mostly within the associated mounting slot 42. The blade platform 46 is outside of the mounting slot 42 but remains in close proximity with the rotor disk 40. Blade platforms 46 of adjacent fan blades 36 align very close to one another. The airfoil 48 extends away from the blade platform until the tip thereof terminates just before reaching the interior surface of the turbofan case 32, as shown in FIG. 1.
During normal operation, the shaft 38 rotates thereby rotating the rotor disk 40. The rotor disk 40 in turn produces the rotation of the fan blade(s) 36 around the shaft 38. However, occasionally the engine experiences a fan blade off event as discussed hereinabove. Upon separation from the shaft 38, the fan blade 36 strikes the case 32. During the fan blade off event, the fan blade travels in both a radial/circumferential and possibly axial, although axial movement may be undesirable, direction away from the turbofan shaft 38. This movement results in the fan blade moving out towards the fan case barrel. The fan blade 36 escaping from the fan case 40 is a safety hazard and may result in damage to the fuselage or wing caused by an impact from the escaped fan blade 36. Therefore, it is an objective of engine design to contain a separated fan blade 36 during a fan blade off event. Some engine certifications are tied to accomplishing this objective.
A need exists for testing turbofan engines and the casings thereof during fan blade off events. Such testing is performed by causing the fan blade(s) 36 to separate from the rotor disk 40 under controlled/observable test conditions. Explosives may be used to cause this separation. Referring now to FIG. 4, a prior art method for separating a fan blade 36 from the rotor disk 40 is depicted. A straight hole 50 is drilled in the thick portion of the fan blade 36 between the blade root 44 and before the blade platform 46. This is referred to as the “stalk” in a conventional blade and is omitted from a blisk style blade. In this illustration, the airfoil 48 is out of view above the platform 46. The thickness of the fan blade 36 proximal the blade root 44 allows for a hole to be drilled near the blade root 44 and likewise easily filled with an explosive charge suitable for causing separation of the fan blade. This is possible because the thickness of the blade stalk allows for the hole 50 to be easily drilled wide enough that ample explosive material may be used to indiscriminately release the fan blade 36 upon detonation or deflagration. Furthermore, in conventional turbofan engines a common failure point is the stalk or root 44 and dovetail joint because this connection point between the fan blade 36 and the rotor disk 40 experiences significant stress during operation.